The Team Tom Jones member triumphed over fellow finalists Bo Bruce (Team Danny O'Donoghue), Tyler James (Team will.i.am) and Vince Kidd (Team Jessie J) to become the first ever victor of the reality show tonight (June 2).
After being asked by host Holly Willoughby for her reaction to the news, a clearly overwhelmed Mitchell said: "Oh my God, I can't believe it."
Upon making his way to the stage to accompany his act, Jones replied: "Well, it's true, and you deserve it. It gives me confidence that there is justice in this world."
Willoughby then asked Mitchell if she had now begun to believe in her vocal abilities, to which the 28-year-old said: "I'm starting to, yeah."
A cloud of confetti then enveloped the stage, with Mitchell's fellow contestants and coaches O'Donoghue, Jessie J and will.i.am running up to congratulate and hug her.
Earlier in the evening, Mitchell had sung versions of James Brown classic 'It's a Man's Man's Man's World' and a reprise of Whitney Houston's 'Run to You', which she had chosen as her favourite song of the series. She also teamed up with her coach on a cover of Jones's 'Mama Told Me Not to Come'.
She will now receive the show's prize of £100,000 and a record deal with Universal Republic.
The night also saw a medley from the coaches on each other's songs, with guest spots from Ed Sheeran, who performed new single 'Small Bump', and Maroon 5, who played both 'Payphone' and 'Moves Like Jagger'.
Watch Leanne Mitchell perform 'It's a Man's Man's Man's World' on the final of The Voice UK below: